Try running on spot and see if you make the same noise. If you don't try running on the spot again, and lean forward until you start moving slowly forward. Think about how your feet hit the ground doing this, what shape your leg is in when it hits the ground doing this and how it compares to how you typically run.
"I was pleased with that but on reflection 2 minutes improvement after all that time and effort-not that great!!"
To be honest it sounds a bit like you're fishing for compliments. After 2000 miles you are going to know exactly what a good and average 5K time is. I mean it's not like you were expecting to drop 5 minutes was it?